Essentially lead free glass and a glass tube made therefrom

ABSTRACT

The invention relates to an essentially lead free glass. The lead free glass is suitable for making glass tubes and glass tubes for electric lamps in particular. The essentially lead free glass is characterized by having the lead oxide of a lead glass replaced by 1.0-3.0% by weight of lithium oxide. The ultraviolet radiation absorption of the glass is improved for wavelengths shorter than 320 nm. by the presence of 0.1-0.3% by weight of cerium oxide (CeO 2 ) and 0.05-0.15% by weight of titanium dioxide (TiO 2 ). With such an essentially lead free glass the transmittance of visible radiation may also be improved.

FIELD OF THE INVENTION

[0001] This invention relates to an essentially lead free glass. Theinvention also relates to a glass tube made from the essentially leadfree glass. The lead free glass has properties making it particularlysuitable for use in tubes of electric lamps. Therefore the glass may beused in the manufacture of light bulbs.

BACKGROUND OF THE INVENTION

[0002] Nowadays glass tubes used in the manufacture of electric lampsare usually made from either a soda-lime glass or a lead glass. The leadglass contains lead oxide. Both lead oxide dust and lead vapour frommelting lead oxide can cause dangerous air pollution harmful to theenvironment and to human beings.

[0003] Lead oxide acts as a fluxing agent and gives lead glass a lowersoftening point than soda-lime glass, It is desirable for glass that isused in making tubes for electric lamps to have this lower softeningpoint but without containing lead oxide.

[0004] For making such a glass suitable for use in electric lampproduction, we have substituted lithium carbonate for the lead oxide.Lithium carbonate is also a fluxing agent. Because of this it allows anincrease in the melting rate of the glass by lowering its viscosity. Italso leads to a lower seed (bubble) count, a lower thermal expansioncoefficient and a higher chemical durability of the glass as well.

[0005] We have also found that when cerium oxide and titanium dioxideare mixed with the essentially lead free glass, a glass may be made thatis capable of absorbing substantially all ultraviolet (UV) radiationhaving a wavelength shorter than 320 nm.

[0006] It has been known that cerium oxide may be used as a decolouriserand a refining agent. It also acts as a flux in glass making and helpsclear gas by reducing seeding and bubble formation in molten glass. Inaddition, the use of cerium oxide has also been reported to increase thelife expectancy of he melting electrodes used in electric furnaces.

[0007] Cerium oxide may have its performance complemented by theaddition of sodium nitrate, which cause a chemical reaction with thecerium oxide.

[0008] The presence of cerium oxide, with or without the addition ofsodium nitrate, does not increase the absorption of the visible spectrumof light and light transmittance at wavelengths greater than 320 nm. isparticularly satisfactory for the manufacturer of electric lamps.

SUMMARY OF THE INVENTION

[0009] An object of at least one preferred embodiment of the presentinvention is the provision of an essentially lead free glass suitablefor the manufacture of tubes for electric lamps.

[0010] Another object of at least a preferred embodiment of theinvention is such an essentially lead free glass tube. This is achievedby replacing the potentially harmful lead oxide present in lead glass.

[0011] Another object of at least a preferred embodiment of theinvention is to provide an essentially lead free glass that absorbssubstantially all UV radiation having a wavelength shorter than 320 nm.while, preferably, increasing the efficiency of transmission of visiblelight. These features are also desirable in glass tubes used in themanufacture of electric lamps.

[0012] In a first aspect, the present invention broadly consists in anessentially lead free glass comprising: Compound Percentage Li₂O 1.0-3.0CeO₂ 0.1-0.3 TiO₂ 0.05-0.15

[0013] The lead free glass will typically comprise 65.0-75.0% SiO₂. Itmay contain one or more, and preferably contains all of the following:2.0-4.0% Al₂O₃, 1.0-3.0% B₂O₃, 3.0-5.0% BaO, a total of 6.0-9.0% MgOand/or CaO, 6.0-9.00% Na₂O, 3.0-5.0% K₂O, 1.0-3.0%. Li₂O, 0.1-0.3% CeO₂,optionally about 0.01% Fe2O3 and 0.05-0.15% TiO₂.

[0014] In a second aspect, the present invention broadly consists in aglass tube made from an essentially lead free glass as defined above.

[0015] In a third aspect, the present invention broadly consists in anelectric lamp having its tube (which term includes “bulb”) made from theessentially lead fee glass as defined above.

[0016] In the specification, unless otherwise stated, percentages are byweight.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a graph showing light transmittance at various wavelengths for an essentially lead fee glass conning 0.01% titanium dioxide(TiO₂).

[0018]FIG. 2 is a graph showing light transmittance at various wavelengths for an essentially lead free glass containing 0.10% titaniumdioxide (TiO₂).

[0019]FIG. 3 is a graph of flexibility or softening point showing theelongation rate per minute at various temperatures.

DESCRIPTION OF THE INVENTION

[0020] The present invention has resulted from a case study made by theInventor with the intention of finding a substitute substance for thelead oxide contained in lead glass while, in general, retaining as muchas possible the desirable properties of lead glass. At the same time,the Inventor desired to improve, if possible, the absorbance of theglass to UV radiation that is harmful to the eyes of human beings.

[0021] The Inventor worked in the production of glass tubing made fromsoda-lime and lead glass. The Inventor discovered that the proportion ofsoda ash ad potassium carbonate, which respectively form sodium oxide(Na₂O) and potassium oxide (K₂O) in the glass, was the main factoraffecting the coefficient of expansion or alpha (α) value of the glass.Therefore, for the essentially lead See glass Of the present inventionit was desired to have the alpha value close to that of lead glass. Fora glass having a sodium oxide (Na₂O) content of 7.5-9.5% and a potassiumoxide (K₂O) content of 3.5-5.0%, an alpha value range of about(93.0-95.0)×10⁻⁷/° C. was obtained. The extent of the thermal expansionof glass is related to its temperature durability. This is relevant forthe use of glass for the tubes of electric lamps.

[0022] It was also necessary to consider the softening point of theglass as well because this is one of the most important properties ofglass. For lead glass containing litharge to provide the lead oxide(PbO) content, the softening point of the glass is lower than that ofsoda-lime glass. The Inventor found that the softening point ofsoda-lime glass was generally in the range 690-700° C., whereas thesoftening point of lead glass was generally in the range of 610-620° C.

[0023] Therefore, instead of the litharge we used a lithium oxideforming substance. In particular, we used 1.5% of lithium carbonate(Li₂CO₃), which formed lithium oxide (Li₂O) in the mix with the otherraw materials for making a glass having a softening point higher thanthat of lead glass but not greater than that of soda-lime glass.

[0024] In addition, we mixed 0.20-0.25% of cerium oxide (CeO₂) andtitanium dioxide (TiO₂) into the glass mixture to produce a glass havingan improved light transmittance; by that meaning a better absorbance atwave lengths shorter than 320 nm. with at least normal and preferablyimproved transmittance of visible light. Normally, titanium dioxide(TiO₂) was an impurity in the mixture and, depending on the degree italready existed in the mixture as an impurity, only 0-0.01% was requiredto be added to the mixture.

[0025] The following describe some examples of the invention. They aregiven only by way of demonstrating the working of the invention and arenot intended to limit the scope of the invention as broadly claimed.

EXAMPLE 1

[0026] The raw material mixture for the essentially lead free glass wasas follows: Compound Percentage SiO₂ 68.67 Al₂O₃ 3.00 B₂O₃ 2.00 BaO 4.00MgO {close oversize brace} 8.00 CaO Na₂O₃ 7.60 K₂O 5.00 Li₂O 1.50 CeO₂0.20 Fe₂O₃ 0.02 TiO₂ 0.01

[0027] To obtain these percentages of the compounds, we calculated theraw materials used in the melting glass method to be as follows: Sand68.85 Soda ash dense 10.80 Alumina 2.97 Potassium Carbonate 7.36Dolomite 15.18 Barium Carbonate 5.24 Lithium Carbonate 3.73 SodiumNitrate 1.00 Cerium Oxide 0.20 Borax Pentahydrate 4.12

[0028] After ensuring that the glass was essentially lead free, wechecked its physical properties and found that it had an alpha (α) valueof 92.0×10⁻⁷/° C. and a softening point of 690° C. For this example, thealpha value was lower than that for lead glass but the softening pointwas higher than that for lead glass but close to that for soda-limeglass.

[0029] Therefore, we adjusted the chemical balance to reduce thesoftening point by changing the amount of lithium oxide from 1.50% to2.70% in the following example.

EXAMPLE 2

[0030] Compound Percentage SiO₂ 68.78 Al₂O₃ 3.00 B₂O₃ 2.00 BaO 4.00 MgO{close oversize brace} 7.80 CaO Na₂O 7.30 K₂O 4.00 Li₂O 2.70 CeO₂ 0.20Fe₂O 0.01 TiO₂ 0.01

[0031] To obtain these percentages of the chemical compounds, wecalculated the proportions of the materials required before performingthe melting glass method as follows: Sand 69.00 Soda ash dense 10.31Alumina 2.98 Potassium Carbonate 5.88 Dolomite 15.19 Barium Carbonate5.24 Lithium Carbonate 6.73 Sodium Nitrate 1.00 Cerium Oxide 0.20 BoraxPentahydrate 4.12

[0032] After ensuring the glass was essentially lead free, weinvestigated its physical properties and found its alpha (α) value to be95.4×10⁻⁷/° C. and its softening point value to be 656° C.

[0033] For the Example 2, the alpha value was higher and the softeningpoint was lower than that of Example 1. We proceeded to investigate thetransmittance properties of Example 2. The result found was that thepercentage transmittance of UV radiation was substantially zero forwavelengths less than 320 nm, but rose for longer wavelengths. At thewavelength becoming visible to human eyes, the percentage transmittanceincreased to about 48.29% (FIG. 1).

[0034] Therefore, to encourage the highest utility in the industriallamp production industry, we added to the essentially lead free glass0.10% of titanium dioxide (TiO₂). We found that this increased apercentage transmittance of the visible light at a wavelength of 350nm., that transmittance being 67.64% (FIG. 2).

[0035] From this, the Inventor concluded that for at least one preferredembodiment of the invention the most suitable chemical proportions to beused in the production of essentially lead free glass are as follows:Compound Percentage SiO₂ 68.89 Al₂O₃ 3.00 B₂O₃ 2.00 BaO 4.00 MgO {closeoversize brace} 7.80 CaO Na₂O 7.80 K₂O 4.00 Li₂O 2.20 CeO₂ 0.20 Fe₂O₃(0.01) TiO₂ 0.10

[0036] In the above table the percentage of Fe₂O₃ represented the levelof the impurity contained in the raw materials. By removing essentiallyall Fe₂O₃ it was found to have no significant effect on any proper ofthe essentially lead free glass.

[0037] The physical properties of this glass were investigated and itwas found to have an alpha (α) value of (94.5±0.5)×10⁻⁷/° C. and asofening point of 660±5° C.

[0038]FIG. 3 is a graph showing the elongation rate of glass per minuteplotted against the temperature and enables the softening point value ofthe glass to be determined. For an elongation rate of 1.0 nm./minute thesoftening point is shown to be 660° C.

[0039] From an examination of other properties of the essentially leadfree glass, we determined its density to be 2.54±0.01 g/cc. This densityvalue is between the density values of soda-lime and lead glasses.

[0040] We have intentionally described the easiest empirical experimentsin the above examples. As already stated, these examples are intended tobe illustrative of the invention only and are not intended to limit thescope of the invention claimed.

1. An essentially lead free glass comprising: Compound Percentage Li₂O1.0-3.0 CeO₂ 0.1-0.3 TiO₂ 0.05-0.15


2. An essentially lead free glass comprising: Compound Percentage SiO₂65.0-75.0 Al₂O₃ 2.0-4.0 B₂O₃ 1.0-3.0 BaO 3.0-5.0 MgO {close oversizebrace} 6.0-9.0 CaO Na₂O 6.0-9.0 K₂O 3.0-5.0 Li₂O 1.0-3.0 CeO₂ 0.1-0.3Fe₂O₃ (0.01) optional TiO₂ 0.05-0.15


3. An essentially lead free glass according to claim 1 wherein the glasshas a softening point between the softening points of soda-lime glassand lead glass.
 4. An essentially lead free glass accordingly to claim 1wherein the glass has a UV transmittance of essentially zero forwavelengths shorter than 320 nm.
 5. An essentially lead-free glassaccording to claim 1 wherein the glass has an improved transmittance oflight at a wavelength of 350 nm. relative to a glass essentially free ofcerium oxide and titanium dioxide.
 6. A tube made from the essentiallylead free glass as claimed in any one of the preceding claims 1-5.
 7. Anelectric lamp having a tube made from the essentially lead free glass asclaimed in any one of claims 1-5.
 8. An essentially lead free glassaccording to claim 2 wherein the glass has a softening point between thesoftening points of soda-lime glass and lead glass.
 9. An essentiallylead free glass according to claim 2 wherein the glass has a UVtransmittance of essentially zero for wavelengths shorter than 320 nm.10. An essentially lead free glass according to claim 2 wherein theglass has an improved transmittance of light at a wavelength of 350 nm.relative to a glass essentially free of cerium oxide and titaniumdioxide.
 11. A tube made from the essentially lead free glass as claimedin any one of claims 2 and 8-10.
 12. An electric lamp having a tube madefrom the essentially lead free glass as claimed in any one of claims 2and 8-10.